Unit test and changelog follow up to batch processor descriptor fix

[dashpole]

This makes sure we have full test coverage for all data types in the batch processor, and adds a changelog entry for the fix in #4389, as requested by reviewers.

[codecov]

Codecov Report

Merging #4396 (d731386) into main (4bb261b) will increase coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4396      +/-   ##
==========================================
+ Coverage   89.52%   89.85%   +0.33%     
==========================================
  Files         176      176              
  Lines       10334    10334              
==========================================
+ Hits         9251     9286      +35     
+ Misses        845      810      -35     
  Partials      238      238              

Impacted Files	Coverage Δ
processor/batchprocessor/splitmetrics.go	98.43% <0.00%> (+27.34%)	⬆️

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